Surface plasmon excitation device and microscope including the same

ABSTRACT

A device for exciting surface plasmons includes a light illuminating source, a transparent substrate having a ridge, a metal layer covering side surfaces of the ridge and their neighboring region, and a thin metal film formed on a top face of the ridge. Evanescent waves caused by light emitted from the light illuminating source and transmitted through the transparent substrate and the thin metal film can excite surface plasmons in the thin metal film.

TECHNICAL FIELD

The present invention relates to a device for exciting surface plasmonsand to improvement in a microscope including the device for excitingsurface plasmons.

BACKGROUND ART

In the conventional field of optics, the focal spot size has beenrestricted due to the diffraction limit of light. In recent years,however, near-field light that can exceed the limit has attractedattention, and investigations using the near-field light have beenvigorously conducted in various fields, for example, using a scanningnear field optical microscope (SNOM) capable of observing an object ofnanometer size. Among the applications of the near-field light, surfaceplasmon resonance has particularly attracted attention, which makes itpossible to obtain electric field intensity of tens of times of that ofincident light. Here, the surface plasmon resonance means a phenomenonthat plasma oscillation of free electrons, which are generated locallyin a metal surface layer when external electromagnetic waves are appliedthereto, comes to resonate with the applied electromagnetic waves.

Japanese Patent Laying-Open No. 1-138443 discloses a device for causingthe surface plasmon resonance. FIG. 6 is a schematic cross sectionalview of a basic device for causing the surface plasmon resonance. Thisdevice includes a light source 101, a light converging lens 102 forconverging light emitted from the light source, a triangular prism 103formed of a transparent dielectric, a thin metal film 104 formed on asurface of the triangular prism, and a photodetector 105 for detectinglight reflected by the thin metal film.

P-polarized light emitted from light source 101 is transformed by lightconversing lens 102 to convergent light, which is transmitted throughtriangular prism 103 and focused on thin metal film 104 at an incidentangle θ. Incidentally, “p-polarized light” means linearly polarizedlight in which the electric vector of the light incident on a substancesurface has a vibration direction that lies in a plane including thetraveling direction of the light and a line normal to the substancesurface. A part of the light focused on thin metal film 104 satisfiesthe resonance condition and comes to resonate with a surface plasmon tocause an enhanced evanescent field 106 on the free surface side of thinmetal film 104. The remaining part of the light is reflected and then isdetected by photodetector 105.

A graph shown in FIG. 7 is obtained by changing the incident angle θ oflight on thin metal film 104. In this graph, a horizontal axisrepresents the incident angle θ of light, and a vertical axis representsreflectance (%). In FIG. 7, the intensity of light received byphotodetector 105 becomes a minimum at a specific incident angle θs,indicating that a part of the convergent light resonates with a surfaceplasmon at that incident angle.

Japanese Patent Laying-Open No. 5-240787 discloses an application of theabove-described device for exciting surface plasmons to a microscope.FIG. 8 is a schematic cross sectional view of a basic microscopeutilizing surface plasmons. This figure shows a light source 201, a beamexpander (lenses 202, 203) for expanding parallel light emitted from thelight source, a light converging lens 204 for transforming the parallellight expanded by the beam expander into convergent light, a prism 205for coupling the light, a thin metal film 206 formed on a surface ofprism 205, a specimen 208 separated from the thin metal film with a gapfilled by emulsion oil 207, a photodetector 209 for detecting the lightreflected by thin metal film 206, and an X-Y pulse stage 210 for movingspecimen 208 intermittently.

Parallel light emitted from light source 201 is expanded by beamexpander 202, 203, and transformed by light converging lens 204 toconvergent light which is then transmitted through prism 205 and focusedon thin metal film 206. Of the focused light, a light part having aspecific incident angle excites a surface plasmon. The incident angledepends on the thicknesses and refractive indices of thin metal film206, emulsion oil 207 and specimen 208.

The light reflected by thin metal film 206, without contributing toexcitation of the surface plasmon, is measured by photodetector 209.Photodetector 209 detects coordinates where intensity of the reflectedlight is reduced due to excitation of the surface plasmon, and then asurface plasmon excitation angle can be obtained from the coordinates,thereby making it possible to determine change in refractive index ofspecimen 208. Further, X-Y pulse stage 210 is used to scan specimen 208so as to obtain two-dimensional distribution of the refractive index ofthe specimen.

With the device shown in FIG. 6 or FIG. 8, however, the area where thesurface plasmon is excited depends on the spot size of the focusedlight. For example, in FIG. 6, if light source 101 has a wavelength of650 nm and light converging lens 102 has an NA (numerical aperture) of0.6, the light beam can be narrowed only to a diameter of about 1 μm.This means that the microscope of FIG. 8 can obtain a resolution only onthe order of 1 μm. That is, the resolution limit of a microscope isdetermined by the diffraction limit of light emitted from the lightsource.

On the other hand, it is possible to reduce the spot size to a certaindegree by decreasing the wavelength of the source light and increasingthe NA of the light converging lens. However, it is extremely difficultto obtain a small light spot of an nm order size. Thus, it appears thatthe resolution of the conventional microscope utilizing surface plasmonshas already reached a critical limit.

DISCLOSURE OF THE INVENTION

In view of the above-described situations of the conventional art, anobject of the present invention is to provide a device that can excitesurface plasmons in a micro area and to provide a high-resolutionmicroscope utilizing the device.

A device for exciting surface plasmons according to the presentinvention includes a light illuminating source, a transparent substratehaving a ridge, a metal layer covering side surfaces of the ridge andthe neighboring regions, and a thin metal film formed on a top face ofthe ridge, wherein evanescent waves caused by light emitted from thelight illuminating source and transmitted through the transparentsubstrate and the thin metal film can excite surface plasmons in thethin metal film.

Preferably, the ridge of the substrate is formed in a striped manner,and the light emitted from the light illuminating source is linearlypolarized in a plane that includes a longitudinal direction and a normaldirection of the top face of the ridge stripe. Still preferably, thelight emitted from the light illuminating source is convergent light.

Preferably, the shape and dimensions and refractive index of the ridge,and the metal layer are set such that light emitted from the lightilluminating source and directed to the ridge reaches the thin metalfilm in an area smaller than a width of the ridge.

Preferably, the metal layer is formed of a conductor, and the thin metalfilm is formed of one of gold, silver, copper and aluminum.

A surface plasmon microscope according to the present invention includesthe device for exciting surface plasmons described above, aphotodetector for receiving light reflected by the thin metal film andthe metal layer included in the device, and a movable support forpositioning a surface of a specimen in the vicinity of the thin metalfilm and for scanning the surface of the specimen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view of a device for excitingsurface plasmons according to an embodiment of the present invention.

FIG. 2 is a schematic perspective view of the device for excitingsurface plasmons according to the embodiment of the present invention.

FIG. 3 is a schematic cross sectional view illustrating an area wherelight can exist in the device for exciting surface plasmons according tothe embodiment of the present invention.

FIG. 4 is a graph showing the relation between the metal groove depthand the reflected light intensity in the device for exciting surfaceplasmons according to the embodiment of the present invention.

FIG. 5 is a schematic cross sectional view of a surface plasmonmicroscope according to another embodiment of the present invention.

FIG. 6 is a cross sectional view of a conventional device for excitingsurface plasmons.

FIG. 7 is a graph showing the relation between the incident angle andthe reflectance of light in the conventional device for exciting surfaceplasmons.

FIG. 8 is a cross sectional view of a conventional surface plasmonmicroscope.

BEST MODES FOR CARRYING OUT THE INVENTION

A device for exciting surface plasmons according to an embodiment of thepresent invention is schematically shown in a cross sectional view inFIG. 1 and in a perspective view in FIG. 2. FIG. 1 corresponds to across section at the x-z plane in FIG. 2. Throughout the figures, thesame reference characters denote the same or corresponding portions.

This device for exciting surface plasmons includes a light illuminatingsource 1, a transparent substrate 2 having a striped ridge, a metallayer 3 formed to cover side surfaces and the neighboring regions of theridge, and a thin metal film 4 formed on a top face of the ridge. Here,metal layer 3 and thin metal film 4 are preferably formed of gold thathardly deteriorates with age and can reduce the propagation distance ofsurface plasmons. Further, for the purpose of exciting surface plasmons,thin metal film 4 is formed to have a thickness such that the incidentlight can be transmitted through thin metal film 4 so as to cause anevanescent field.

The convergent light emitted from light illuminating source 1 andlinearly polarized in the x-z plane is transmitted through substrate 2and enters the inside of the ridge. At this time, the light emitted fromlight illuminating source 1 is set to have an incident angle θs withwhich the surface plasmon is excited in thin metal film 4 formed on thetop face of the ridge. Further, the cross sectional shape and dimensionsof the striped ridge, and the materials of metal layer 3 and thin metalfilm 4 are set such that the incident light reaches a micro area on thinmetal film 4. The light having reached the micro area of thin metal film4 formed on top of the ridge and linearly polarized in the x-z planecauses evanescent waves 5 enhanced by the surface plasmon resonance.These evanescent waves propagate in the −x direction along the surfaceof thin metal film 4 on the side exposed to the air.

Here, specific explanation is given as to how the shape and dimensionsof the ridge are determined. In FIG. 3, shaded part in the ridgerepresents an area where light can exist in the case that light linearlypolarized in the x-z plane is incident on substrate 2. As seen from thisfigure, when the light linearly polarized in the x-z plane enters theinside of the ridge having its side surfaces covered with metal layers3, the electric field parallel to the side surface “a” of the metallayer cannot exist in the vicinity of the side surface “a”. Further, inthe vicinity of side surface “a” of the metal layer, the area where thelight can exist decreases as the light advances in the −z direction.Still further, if the depth “d” inside the ridge is greater than acertain value, the light can enter only partway inside the ridge.

Such a phenomenon can be confirmed from the following analysis. Thegraph in FIG. 4 shows the relation between the depth of the metal grooveand the intensity of reflected light with respect to the incidentpolarized light in the case that the substrate has a refractive index nof 1.58, the metal groove has a width of 250 nm, the medium forming themetal groove is gold, the incident light has a wavelength λ of 650 nm,the objective lens has an NA of 0.6, and the incident angle of light tothe bottom of the metal groove is 0° (i.e., perpendicular to thesubstrate surface). In this graph, a horizontal axis represents thedepth of the groove normalized with λ/n, and a vertical axis representsthe intensity of the reflected light normalized with the intensity ofthe incident light. Although the medium forming the metal groove is goldin FIG. 4, the graph will show similar tendencies even if another metalsuch as silver, copper, aluminum or the like is employed.

Referring to FIG. 4, in the case of the light (shown with the symbol ♦in the graph) polarized in the direction perpendicular to the sidesurfaces of the groove, the electric field of the incident light canexist in the vicinity of the side surfaces of the metal groove. Thus,there occurs a relative phase difference between the reflected lightfrom the top level part of the groove and the reflected light from thebottom level part of the groove depending on the depth of the groove,and as a result, the intensity of the reflected light changes due tointerference of the lights. By comparison, in the case of the light(shown with the symbol ▪ in the graph) polarized in the directionparallel to the side surface of the metal groove, it is seen that theintensity of the reflected light becomes almost constant with the groovedepth exceeding 0.3 λ/n. This means that, when the metal groove is deep,the light polarized parallel to the side surface of the metal grooveenters only to the depth of about 0.3 λ/n.

Thus, it is possible to form a ridge allowing the incident light toreach only a micro area w1 of thin metal film 4 by setting, e.g., thewavelength λ of light illuminating source 1 to 650 nm and the height dand the width w of the ridge to 0.3 λ/n and 250 nm, respectively.Although the incident angle is 0° in this example, it is possible toform, even with the incident angle of θs, a ridge that allows theincident light to reach only a micro area w1 of thin metal film 4.

Accordingly, when the incident light is polarized in the x-z plane,surface plasmons can be excited in thin metal film 4, at a desiredheight d of the ridge and in an area smaller than the width of theridge. However, if the light polarized perpendicular to the x-z planeenters, it will reach over the entire width of thin metal film 4 on thetop face of the ridge, in which case it is not possible to excite thesurface plasmons in thin metal film 4 in an area smaller than the widthof the ridge.

As understood from the above description, according to the presentinvention, it is possible to excite surface plasmons within a widthsmaller than that of the ridge, irrespective of the width of theincident light. When this is applied to a surface plasmon microscope forexample, it is possible to improve the resolution of the microscope.

The schematic cross sectional view shown in FIG. 5 illustrates a surfaceplasmon microscope according to another embodiment of the presentinvention. This figure shows a light illuminating source 1, a substrate2 having a striped ridge, a metal layer 3 formed to cover side surfacesof the ridge, a thin metal film 4 formed on a top face of the ridge, aphotodetector 8 for detecting light reflected by metal layer 3 and thinmetal film 4, a specimen 6 spaced from thin metal film 4 with a gapfilled with matching oil (not shown), and a movable stage 7 for scanningthe specimen.

The convergent light emitted from light illuminating source 1 istransmitted through substrate 2 and focused on thin metal film 4. Of thefocused light, a light part having a specific incident angle, whichdepends on the thicknesses and refractive indices of thin metal film 4,emulsion oil and specimen 6, excites a surface plasmon. Photodetector 8detects light reflected by metal layer 3 and thin metal film 4 and notcontributing to excitation of the surface plasmon. The coordinates atwhich the intensity of reflected light is reduced due to the surfaceplasmon excitation is detected on photodetector 8 and the surfaceplasmon excitation angle is calculated from the coordinates, to therebyobtain the refractive index of specimen 6.

Further, movable stage 7 is employed to scan specimen 6 so as to obtainthe two-dimensional distribution of the refractive index. For example,if there is a micro area 6 a in specimen 6 having a locally differentrefractive index, then the position of that area can be detected. Atthis time, a surface plasmon is excited by the device for excitingsurface plasmons only in a restricted area having a width smaller thanthat of the ridge by virtue of the structure of the ridge, andaccordingly, it is possible to improve the resolution compared to aconventional microscope.

Although the device for exciting surface plasmons having the stripedridge has been described in the above embodiments, a ridge having itslength limited in the x direction, such as an information pit of anoptical disk, may be utilized as well. In this case, the propagationdistance of the surface plasmon (or of the enhanced evanescent waves)can be restricted, and thus, the area where the surface plasmon isexcited can further be reduced in size in the x direction.

Further, in the above-described embodiments, the height d of the ridgehaving the rectangular cross section is changed to control the size ofthe area where the light reaches the thin metal film formed on the topface of the ridge. Alternatively, the width w of the ridge, metalmedium, cross sectional shape of the ridge and the like may be changedto control the same.

Furthermore, according to the present invention, it is possible toreduce the size of the area where the surface plasmon is generated.Thus, its application to a sensor measuring a change in refractiveindex, for example, enables sensing of a micro area. Still further, theridges may be arranged in an array and the specimens also in acorresponding array, to enable measurements of changes of refractiveindices in micro areas of a great number of specimens. The presentinvention may further be applied to a device for measuring fluorescentreaction such as a biochip, to detect fluorescence of objects arrangedin high density in a micro area, as in the case of a sensor.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, it is possibleto provide a device that can excite surface plasmons in a micro area andto provide a high-resolution microscope utilizing that device.

1. A device for exciting surface plasmons, including light illuminatingmeans, a transparent substrate having a ridge formed in a stripedmanner, a metal layer covering side surfaces of said ridge and theirneighboring region, and a thin metal film formed on a top surface ofsaid ridge, wherein said light illuminating means is arranged so as toilluminate said ridge with light which is linearly polarized in a planethat includes a longitudinal direction and a normal direction of the topsurface of said striped ridge, said metal layer is provided forgradually narrowing a width of said light advancing in a depth directionin said ridge, and evanescent waves caused by said light are transmittedthrough said transparent substrate and said thin metal film can excitesurface plasmons in said thin metal film.
 2. The device for excitingsurface plasmons according to claim 1, wherein the light emitted fromsaid light illuminating means is convergent light.
 3. The device forexciting surface plasmons according to claim 1, wherein shape anddimensions and refractive index of said ridge, and said metal layer areset such that the light emitted from said light illuminating means anddirected to said ridge reaches said thin metal film in an area smallerthan a width of said ridge.
 4. The device for exciting surface plasmonsaccording to claim 1, wherein said metal layer is formed of a conductor,and said thin metal film is formed of one of gold, silver, copper andaluminum.
 5. A surface plasmon microscope, including the device forexciting surface plasmons as recited in claim 1, a photodetector forreceiving light reflected by said thin metal film and said metal layer,and movable support means for positioning a surface of a specimen in thevicinity of said thin metal film and for scanning the surface of thespecimen.